ARM: dts: imx53-tx53: remove the regulators bus

It is not recommended to place the regulator nodes inside 'simple-bus',
so adjust them accordingly.

The motivation for rearranging this is to make it easier to add new
regulator nodes in the future.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>